DNA damage accumulates during life and is thought to contribute to aging and genomic instability. Therefore, defining those proteins and pathways that maintain genomic stability is critical in preventing aging and age-related degeneration. This project aims to understand what roles the five human RecQ proteins play in DNA repair and genomic stability. RecQ proteins play fundamental roles in several DNA metabolic pathways including DNA double-strand break repair (DSBR), mainly in the form of homologous recombination (HR), non-homologous endjoining (NHEJ) and replication. One major goal of our lab is to delineate the unique and complementary roles of the human RecQ helicases, with the expectation that we might fully explain the function of each protein. Each RecQ possesses helicase and strand annealing activity as well as domains that confer unique functionality. We have demonstrated that all five human RecQ helicases can be recruited to laser-induced double strand breaks (DSB), however the details of how each RecQ participates in DSB is less clear and therefore efforts are ongoing to dissect more precisely the role of each RecQ in DSB repair. RECQL4 is highly expressed during S phase when HR-dependent DSBR is active. Previoulsy, we found that RECQL4-deficient cells have a profound defect in HR-mediated DSBR. We showed that RECQL4 interacts with the MRN/CtIP complex, and that its helicase activity is required for DNA end-resection during DSBR. Moving forward we are continuing to investigate the interactions of RECQL4 with other DSBR proteins. One abundant interaction partner for RecQs that is of interest is Poly(ADP-ribose) polymerase (PARP1). PARP1 and the polyADP-ribose polymer (PAR) play important roles in the response to DNA damage. PARP1 is a NAD+-dependent DNA damage-sensor that covalently attaches multiple ADP-ribose moieties onto its protein substrates, including itself. Our experiments suggest that PARP1 and/or PAR play a role in recruiting WRN and RECQL5 to DSBs, as their recruitment is delayed in PARP1-deficient cells or in cells treated with PARP1 inhibitors. PARP1 stimulates the strand annealing activity and inhibited the helicase activity of RECQL4. RECQL4 is the helicase with the strongest stand annealing among the five human RecQ proteins, therefore we are exploring if RECQL4 functions in DNA repair pathways that utilize strand annealing preferentially. Our findings have implications on how cells decide which DSBR subpathway is active at any given DSB (i.e., pathway choice) thus we are continuing to characterize the role of RecQ proteins in DSBR pathway choice.